1. Field of the Invention
The present invention relates to a MO-PA (Master Oscillator-Power Amp or Oscillator+Amplifier) type optical amplifier or laser in which an oscillator and an amplifier having a fiber for optical amplification are combined. More specifically, it relates to an optical amplifier that can prevent damage or breakage of an oscillator caused by reflected-light, a fiber laser using the same, and a method of eliminating reflected-light in a MO-PA type optical amplifier.
2. Description of the Related Art
Recently, attention has been drawn to fiber lasers which are used as a light source for diagnosis, measurement, remedy, fabrication and the like. Among other fiber lasers, a pulse laser is widely used as a light source for fabrication and a light source for measurement.
When a laser for pulse oscillation is formed by a fiber laser, there are many cases in which they are structured in a MO-PA type. The MO-PA type lasers have the advantage that since in each laser an oscillator and an amplifier can be separately formed from each other, the oscillator can be formed by a number of optical components for low power and the amplifier can be formed by a small number of optical components for high power. Namely, such a use of different components for different purposes is advantageous since the costs of optical components for high power are generally higher than those of optical components for low power. Another advantage of the MO-PA type is that an amplifier in which the insertion loss of optical components has a significant effect on an optical gain is formed separately from an oscillator in which high-loss optical components must be used, and whereby a high-output power in totality can effectively be obtained.
However, such a MO-PA type high-power optical amplifier and a MO-PA type fiber laser have a drawback in that they are weak or vulnerable to reflected-light. When reflected-light which has returned to the optical amplifier or fiber laser and which may be weak is passed in a reverse direction through a PA section (amplifying section) inside an apparatus, it is amplified or receives a large power amplification, and therefore, there is a concern that a MO section would be destroyed due to this kind of large power. Conventionally, as a means to reduce such power, an optical isolator is used in some cases. However, it is necessary for an optical isolator to have a power-resistant characteristic when it is used in a laser. Consequently, the optical isolator is generally large-sized and high-priced, resulting in an increase in optical loss. Incidentally, although the inventors considered a method of minimizing the effects of reflected-light by the use of a control circuit or the like, the control became complicated and it was difficult to control reflection under all circumstances.